THYROID GLAND:
Origin of Thyroid Gland:
It develops from the endoderm of the embryo.
Location and Structure of Thyroid Gland:
The thyroid gland is the largest endocrine gland located anterior to the thyroid cartilage of the larynx in the neck. The gland is well supplied with blood vessels. It is bilobed organ. The two lobes are connected by a narrow structure called the isthmus. The microscopic structure of the thyroid gland shows thyroid follicles com¬posed of cubical epithelium and filled with a homogenous material called colloid.
Small amount of loose connective tissue forms stroma of the gland. Besides containing blood capillaries, the stroma contains small clusters of specialized Para follicular cells or ‘C’ cells. The thyroid gland is the only gland that stores hormones in large quantities for about two months.
Hormones of Thyroid Gland:
The thyroid gland secretes three hormones. Thyroxine (tetraiodothyronine or T4), and tri¬iodothyronine or T3 are secreted by the thyroid follicular cells. Thyrocalcitonin is secreted by the ะก-cells of the thyroid gland. This gland is stimulated to secrete its hor¬mones by thyroid stimulating hormone (also called thyrotropin) secreted by the anterior lobe of pituitary gland.
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(i) Thyroxine (T4) and Tri¬iodothyronine (T3):
T4 and T3 contain four and three atoms of iodine respectively, there¬fore, they are named so. T3 is secreted in smaller amounts but it is more active and several times more potent than T4. T4 is converted to T3 by removal of one iodine in the liver, kidneys and some other tissues. Since both T4 and T3have similar effects on the target cells, they are generally con-sidered together under the name, thyroid hormone (TH).
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